Volcanoes by Matthew Spiteri 3.02

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Volcanoes by Matthew Spiteri 3.02

  1. 1. Composite volcanoes, also called Strato volcanoes, they areformed by alternating layers of lava and rock fragments. Thisis the reason they are called composite. Strato-volcanoes often form impressive, snow-capped peakswhich are often exceeding 2500m in height, 1000sq.km insurface, and 400km3 in volume. Between eruptions they are often so quiet they seemextinct. To witness the start of a great eruption requires luckor very careful surveillance. Composite volcanoes usually erupt in an explosive way. Thisis usually caused by viscous magma. When very viscous magmarises to the surface, it usually clogs the crater pipe, and gas inthe crater pipe gets locked up. Therefore, the pressure willincrease resulting in an explosive eruption. Although Strato-volcanoes are usually large and conical, wecan distinguish different shapes of them: concave , pyramidal,convex-concave, helmet-shaped , collapse caldera, nested ,multiple summits, elongated along a fissure .
  2. 2. Different Shapes of Composite Volcanoes.
  3. 3. Shield volcanoes are huge insize. They are built by many layersof runny lava flows. Lava spills out ofa central vent or group of vents. Abroad shaped, gently sloping cone isformed. This is caused by the veryfluid, basaltic lava which cant bepiled up into steep mounds.
  4. 4. Shield volcanoes may be produced by hot spotswhich lay far away from the edges of tectonicplates. Shields also occur along the mid-oceanicridge, where sea-floor spreading is in progressand along subduction related volcanic arcs. The eruptions of shield volcanoes arecharacterized by low-explosivity lava-fountainingthat forms cinder cones and spatter cones at thevent. Famous shield volcanoes can be found forexample in Hawaii.
  5. 5. A Cinder cone is a steep conical hill formed above avent. Cinder cones are among the most common volcaniclandforms found in the world. They arent famous as theireruptions usually dont cause any loss of life. Cinder conesare chiefly formed by Strombolian eruptions. The conesusually grow up in groups and they often occur on the flanksof Strato volcanoes and shield volcanoes. Cinder cones are built from lava fragments calledcinders. The lava fragments are ejected from a single ventand accumulate around the vent when they fall back toearth. Cinder cones grow rapidly and soon approach theirmaximum size. They rarely exceed 250m in height and500m in diameter.
  6. 6. The shape of a cinder cone can bemodified during its (short)life. When the position of the ventalters, aligned, twin or secant conesdevelop. Nested, buried orbreached cones are formed whenthe power of the eruption varies.
  7. 7. When hot erupting lava contains justenough explosive gas to prevent theformation of a lava flow, but not enoughto shatter it into small fragments thelava is torn by expanding gases into fluidhot clots, ranging in size from 1cm to50cm across, called spatter. When the spatter falls back to Earththe clots weld themselves together andsolidify forming steep-sidedaccumulations. These accumulationsfocused on an individual vent are calledspatter cones.
  8. 8. Different shapes of Spatter Cones
  9. 9. Complex Volcanoes can also be called ascompound volcanoes In fact all volcanoes could be complexvolcanoes since all of them are made upof multiple flows, ashlayers, domes, cones, etc. in varyingamounts. However, when we call a volcano acomplex volcano it is because we meanthe "system" of those volcanoes is not"simple". Caldera complexes for instancehave often got a large caldera with manysubsidiary vents and deposits, some ofwhich could be considered "volcanoes" intheir own right. Frankly speaking, a volcano thatconsists of a complex of two or morevents is reckoned as a compound orcomplex volcano.
  10. 10. These are all different types of Complex Volcanoes
  11. 11. How does Volcanoes erupt ???????An eruption begins when pressure on amagma chamber forces magma upthrough the conduit and out thevolcanos vents. When the magmachamber has been completely filled, thetype of eruption partly depends on theamount of gases and silica in themagma. The amount of silica determineshow sticky (level of viscosity) the magmais and water provides the explosivepotential of steam.
  12. 12. 1. low water, low silica - runny lava flows (not viscous)
  13. 13. 2. low water, high silica (very viscous) - pasty lava - often building domes.
  14. 14. 3. high water, low silica(not viscous) - fountain of runny lava
  15. 15. 4. high water, high silica (very viscous) - explosion

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